Synthesis and crystal structure.
Polycrystalline sample of NdTa\(_7\) O\(_{19}\)  was prepared by conventional solid state synthesis method from high purity Nd\(_2\) O\(_3\)  (Alfa Aesar, 99.994%) and Ta\(_2\) O\(_5\)  (Alfa Aesar, 99.993%) [1]}.
Prior to use, Nd\(_2\) O\(_3\)  was preheated at 900\(^\circ \) C for 12 h.
Stoichiometric amounts of each material were mixed and pelletized.
Single phase NdTa\(_7\) O\(_{19}\)  sample was obtained after heat treatment of the reactants for several days at 950\(^\circ \) C, 1000\(^\circ \) C, 1050\(^\circ \) C, 1100\(^\circ \) C, 1150\(^\circ \) C, and 1200\(^\circ \) C with intermediate grindings.
Its phase purity was checked with Rigaku X-Ray diffractometer at room
temperature using Cu \(K_\alpha \)  radiation.
The Rietveld refinement was carried out within the space group \(P\bar{6}c2\)  using fullprof suite [2]}.
The results are shown in Supplementary Fig. 1 and the parameters are summarized in Supplementary Table I.
No anti-site disorder was detected in this compound.
